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UCSB EEMB 2 - Lec #15 EEMB 2 SM17 1S (Speciation)

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The Evolutionary Process:1. Variation2. Natural Selection 3. Genetic Divergence 4. Reproductive Isolation5. SpeciationVariationNatural SelectionReproductive isolationGenetic DivergenceSpeciationSpeciationSpeciationSpeciation: (1.75 million species have been identified) Changes in allele frequencies that are significant enough to mark the formation of daughter species from parent speciesSpecies: How do we define a species?Sp ASp B Sp CSp FSp DSp EPhylogeny: line of descentEx: Poison dart frogs (Dendrobates pumilio), N & S Leopard frogsSpeciationSpeciation: 1. Biological Species Concept: (Ernst Mayer 1942)- Species are groups of interbreeding natural populations that are reproductively isolated form other such groups- the capacity to contribute to a shared gene pool is qualification for species membership- Limitations: this concept does not apply to asexually reproducing organisms (bacteria, some species of plants and fungi)Ex:Western MeadowlarkEastern MeadowlarkSpeciationSpeciation: Ex: Bacteria, Butterflies 2. Morphological species concept:- based on morphological (structural) characteristics - can be applied to sexual and asexual organismsLimitations: Variation in morphology, ontogenetic niche shifts, sexual dimorphism makes this definition difficult3. Paleontological Species Concept: - Species are morphological discrete groups known only from the fossil record - Limitations: limited evidence, not all organisms fossilize wellSpeciationSpeciation: 4. Ecological Species Concept: (Leigh Van Valen - 1976)- Species are separated by their ecological niche- Limitations: niche is hard to define and often flexible5. Phylogenetic Species Concept:- Species are sets of organisms with a unique genetic history - compares physical characteristics & molecular sequences with those of other organisms - limitations: at what % difference do we define a speciesIt is not easy to define a species and there are multiple techniques to do so (Method dependent on species under investigation)SpeciationMechanisms of Speciation:Speciation due to accumulation of genetic changes that promote significant differences between populations (Depends on how gene flow between populations is interrupted)Primary mechanisms of speciation:1. speciation due to abrupt events - Ex: changes in chromosomal # 2. speciation due to adaptation to different environments (RIMs)SpeciationMechanisms of Speciation:1. Allopatric Speciation: Gene flow is interrupted when a physical barrier between populations occurs that promotes the genetic changes necessary for speciation- Physical barrier is effective at blocking gene flow between pops and depends on organisms ability to migrate- Reproductive isolation arises in the genetically diverging populations - Speciation is complete when individuals of the 2 populations can not interbreed (mutation, natural selection, & genetic drift promote genetic divergence)SpeciationMechanisms of Speciation:1. Allopatric Speciation: Physical barriers lead to reproductive isolation Small isolated populations are more likely than large populations to experience allopatric speciation & adaptive radiation (mutation, natural selection, & genetic drift)Ex: Galapagos Islands Ex: Hawaiian Honey CreepersAdaptive radiation: Upon introduction to various new environments, ancestral species has evolved into wide array of descendent species that differ in morphology, physiology and behaviorSpeciationThe Pace of Allopatric Speciation: Timing of Speciation - Measurable geographic distances separate the populations of most species - Gene flow among populations is more an intermittent trickle than a steady stream Rapid Speciation (punctuated equilibria): Barriers arise rapidly and block immigration/emigration between pops.Ex: Earthquake in 1800’s (Mississippi River shift in course)Gradual Speciation (gradualism): Barriers arise gradually & block immigration/emigration between pops., determined by fossils and break up of contiguous habitatsEx: Ice ages (glaciation and interglaciation), rise of mountain rangesSpeciationMechanisms of Speciation:2. Sympatric Speciation: A species may form within the home range of an existing species in the absence of physical barriers (polyploidy, adaptation to local environments, sexual selection)Polyploidy: During cell division an accident can result in extra sets of chromosomes(results in immediate reproductive isolation, relatively common in plants) - Autopolyploidy: Non-disjunction of chromosomes during gamete formation in the same species - an individual that has more than 2 chromosome sets all derived from a single speciesMechanisms of Speciation: Polyploidy (Autopolyploidy) Intraspecies breeding produces 2N (diploid) to 4N (tetraploid) plants via non-disjunction of chromosomes New speciesPolyploidy (Allopolyploidy) - commonEx: Interspecies breeding produces a hybrid (Usually sterile)- Hybrids (1 set of chromosomes from each species) are sterile or infertile with each other but can not interbreed with either parental species - sterile hybrids may propagate asexually and generate speciation- or in subsequent generations, a sterile hybrid can change into a fertile polyploid (allopolyploidy can generate speciation)Sterile Hybrid New species- 2N & 4N cross produces 3N offspring that are sterile (Repro. Isolation) - 4N plants produce fertile offspring via self pollination or mating with other 4N individuals- in one generation, autopolyploidy can generate speciationEx:SpeciationSympatric Speciation: PolyploidyAllopolyploidy: Many Agricultural Crops: (Oats, cotton, potatoes, tobacco, wheat) - All are polyploids- Wheat (Triticum aestivum) is an allohexaploid (6 sets of chromosome)Polyploidy in animals: (rare event, some frogs and fish) Other mechanisms of sympatric speciation: - Genetic factors enable a subpopulation to exploit a resource not used by the parent population (over time genetic differences accumulate leading to reproductive isolation and speciation)SpeciationSympatric Speciation: Adaptation and sexual selectionParapatric speciation: adaptation to local environments and sexual selection promote speciation- Neighboring populations become distinct species while maintaining contact along a common border-Interbreeding individuals produce hybrid offspring in the border (hybrid zone)Ex: African cichlid fishes in Rift LakesUse the variety of species concepts to define when a species


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UCSB EEMB 2 - Lec #15 EEMB 2 SM17 1S (Speciation)

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